Mapback is a term used by paperback collectors to refer to the earliest paperback books published by Dell Books, beginning in 1943. The books are known as mapbacks because the back cover of the book contains a map that illustrates the location of the action. Dell books were numbered in series. Mapbacks extend from #5 to at least #550; then maps became less of a fixed feature of the books and disappeared entirely in 1951. (Numbers 1 through 4 had no map, although a later re-publication of #4, The American Gun Mystery by Ellery Queen, added a map.) The occasional number in the series between #5 and #550 contains no map, but some sort of full-page graphic or text connected with the book's contents.
The artwork of the maps began with quite detailed maps, and later numbers contain more stylized ones. "The back cover map was very popular with readers and remains popular with collectors ... the Dell 'mapbacks' are among the most well known vintage paperbacks."[1]
"Dell's most memorable design innovation was not on the front but on the back covers ... the entire back covers given over to maps, or variously charts, blueprints, or what have you to represent story locale or scene of the crime: a stretch of California highway, the interior of an apartment, a sheik's 'city of stones.' It was an enjoyable if slightly goofy gimmick and, amazingly, managed to last nearly ten years."[2]